A centrifuge instrument is a device by which liquid samples may be subjected to centrifugal forces. The sample is carried within a member known as a centrifuge rotor. The rotor is mounted to a rotatable drive shaft that is connected to a source of motive energy.
Centrifuges currently employed in laboratories are generally operated by manual controls using various settings and procedures. A rotor control may be used to set the centrifuge to a specific size or type of rotor. A temperature control and timer are also frequently used depending on the type of sample being tested. There are conventional power switches to manually turn the units on or off as needed. A physical key lock is commonly used to secure centrifuge access from operation. Screwdriver adjusted sensors or trimmers are traditionally used to correct the setting of an imbalance detector. This adjustment is exceedingly time consuming and inaccurate.
Centrifuge operation presents a unique set of design criteria where precision control of the rotational operation of the centrifuge is required. The wide variety of biological and chemical experimental research which use centrifugation as their primary tool to achieve component separation and perform experimental assays places a requirement of versatility on the operational characteristics which must be built into the centrifuge.
The centrifuge rotor is driven to extremely high rotational speeds in order to generate the centrifugal field required for biological research use. The high rotational speed requires the samples that are placed in the centrifuge rotor to be equally loaded or balanced to a pre-determined level. If this is not done or a sample bottle breaks or leaks at speed a large imbalance will result. Large imbalances at high speed are very destructive to the centrifuge drive and suspension system, and in severe cases cause unwanted centrifuge movement. Centrifuge systems are routinely designed with imbalance detectors to shut down the run in severe imbalance conditions.
The centrifuge apparatus has numerous rotors which may be interchangeably used in conjunction with the same centrifuge motor and drive shaft in order that a diversity of biological experimentation may be achieved. One standard of centrifuge design encompasses a motor with a flexible shaft which can accommodate the interchangeable rotors to be carried on the spindle of the motor shaft, each rotor having a different weight and strength of material and a different maximum safe speed above which the particular rotor should not be operated. Some centrifuge systems will use a flexibly mounted drive shaft coupled to the motor, commonly called a gyro system. This performs the same function as the motor with a flexible shaft, but is more damage tolerant.
Also, an imbalance of the rotor or load which it carries will increase in force as the rotor speed increases, the increase in force will be proportional to the square of the increase in speed. Often, these imbalances do not arise until the rotor has achieved very high speeds, normally through sample tube leakage or breakage. The dynamic effect of large unbalancing forces cause complicated movement of the shaft, or gyro, upon which the rotor is suspended, such as dangerous whirls and gyrations.
The rotor is part of a centrifuge system that includes a motor or other source of motive energy, a drive shaft or gyro system, and a rotor mounting device disposed at the upper end of the drive shaft or gyro shaft, on which the rotor is received. Like other mechanical devices or bodies that rotate at a high speed, the rotor has certain modes of vibration which become apparent when the rotor is accelerated through its speed range. The rotor normally rotates about its geometric center of gravity. At critical speed the rotor shifts its axis of rotation laterally from that of the rotors geometric center to that of the rotors center of mass. During normal use, the rotor generally passes through its critical speed when accelerating from a stopped position to its operating speed, and after centrifugation is completed, when decelerating from its operating speed to a stopped position. Although the rotational energy of the system is low relative to the energy at much higher operating speeds, it is at this lower, so called “critical speed” of rotation that imbalances in the rotor introduce gross loading errors which tend to cause large rotor movements.
Typically, therefore, the centrifuge's drive system mount design including the shaft stiffness or gyro is provided with some form of compliance mechanism which accommodates the forces generated by the system as the rotor's rotation approaches and traverses its critical speed. This system compliance is also designed to operate at high speeds when the system accelerations are at a maximum.
When an operator is loading a centrifuge rotor, an important objective is achieving a weight-balanced, symmetrical sample distribution pattern about the drive shaft and instructions are normally provided to reach this objective. However even the most careful operator will still make errors in loading and sample container leakage and breakage will unexpectedly occur, so the system must be designed to detect when vibration levels are excessive and shut down the centrifuge before damage can occur. If not reduced or sufficiently dampened, the total unbalancing forces, arising from inherent rotor imbalances and/or sample loading patterns may result in premature failure of the centrifuge.
It is therefore clear that a versatile centrifugation system requires: (1) a maximum safe rotor speed be identified for each rotor; (2) the operational use and control of the rotor during centrifugation be monitored and controlled; and, (3) that any imbalance be detected. If possible, the use of a single sensor or transducer system would provide accuracy and asynchronous information which may be used to limit imbalance forces for all varieties of rotors at different speeds.
In laboratory centrifuges, the rotor, i.e., the part of the structure which rotates and which carries a vessel with material to be subjected to centrifugal force, is balanced at the time of manufacture. Nevertheless, in the event of a defect or of uneven loading of the vessel, an imbalance may arise that can be tolerated only within specific limits. Otherwise, damage may occur when operating the centrifuge, especially at high speeds.
Accordingly, centrifuges of this kind are equipped with shutoff devices for turning the motor off when an upper threshold imbalance, empirically ascertained for the particular centrifuge, is exceeded. However, ascertaining the imbalance arising at centrifuge startup entails difficulties.
The state of the art comprises high cost shutoff devices which typically operate using magnetic-field detectors to monitor the rotor-generated magnetic fields and to thereby determine the imbalance.
Known centrifuges have been marketed for many years in which a mechanical switch is mounted on the housing and, upon rotor imbalance and lateral deviation, the switch makes contact with an element mounted on the stator which then actuates the shutoff switch. This design, however, incurs two substantive drawbacks. On one hand, mechanical switches may fail per se and on the other hand the switch or the element on the drive system must be adjusted to assure that switching off takes place accurately at the specific threshold imbalance. Assembly costs are raised as a result. Furthermore, the deviation depends on support tolerances and therefore will differ among units of the same type at the same imbalance.
The present invention overcomes the prior art problems by utilizing an automated centrifuge imbalance detector process and control system.